Recently, with the development of electronic industries and electric industries, a mounting method of printed substrates on instruments for public use and communication has been required to be simplified, miniaturized, and to have a high reliability, and also a flexible printed substrate which can be miniaturized and light-weighted and can make surface-loading has been required to have a high performance. As such a flexible printed substrate for meeting the requirement of such a high performance, a single sided substrate, a double sided substrate, or a multilayer substrate having a copper foil and an insulating resin film directly laminated thereon without using an adhesive has been marketed. Also, for attaining higher density printing and a higher performance, a demand for loading and mounting flexible printed substrates onto an external rigid substrate or a lead frame in a hybrid state has been increased.
However, at present, for loading a flexible printed substrate on an external substrate, a method of previously forming an adhesive layer on the external substrate side or a method of forming an adhesive layer on a flexible printed substrate has been employed.
In the former method, it is necessary to prepare an adhesive film having a shape coinciding with the adhesive region on an external substrate and also in the case of directly coating an external substrate with an adhesive, a precise coating technique is required. Thus, in any case, the above-described step is complicated.
On the other hand, in the latter method, it is necessary to form the adhesive layer on the printed substrate at the formation of the printed substrate, whereby in the case of using a conventionally used thermosetting adhesive comprising an epoxy series resin or a maleimide series resin, there is a possibility of lowering the adhesive characteristics by causing the deterioration of the resin by the contact with an acid or an alkali at a patterning step or a plating step of a wiring circuit. In particular, such a tendency is remarkable in the case of using the above-described thermosetting adhesive, which becomes a serious factor of lowering the reliability at loading and after loading the printed substrate.
On the other hand, as resins having higher heat resistance and chemical resistance than the above-described conventional adhesives, there are polyimide resins and it is disclosed in JP-A-61-22937 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") that in these resins, a thermoplastic polyimide resin has an adhesive function. However, in the method described in the JP-A-61-22937, the thermoplastic polyimide resin layer is in direct contact with a copper foil and hence there are some problems in the point of chemical resistance at patterning a wiring circuit, etc., and also since the surface of a polyimide resin layer is usually inactive, in the case of laminating a thermosetting polyimide resin layer and a thermoplastic polyimide resin layer, the adhesive property is poor at the interface between the layers and hence a surface treatment is required for improving the adhesive power, which complicates the production step.
Also, recently, for obtaining high density printing and a high performance, the development and requirement for a double sided substrate from a single sided substrate have been increased and for meeting the requirement, double sided substrates having various structures have been proposed. As an example of such a double sided substrate, there is proposed a double sided substrate prepared by press-bonding under heating a copper foil onto both surfaces of a thermoplastic polyimide resin layer as an insulating resin substrate. However, since such a thermoplastic polyimide resin is inferior a little in heat resistance, chemical resistance, and dimensional stability to a conventionally used thermosetting polyimide resin, such a double sided substrate is insufficient for practical use.
Also, a linear expansion coefficient of a thermoplastic polyimide resin is generally from about 2 to 4 times the linear expansion coefficient of a copper foil and thus when a wiring circuit is patterned at the copper foils formed on both surfaces of the resin substrate, there is a possibility of causing curling of the substrate.
On the other hand, if a thermosetting polyimide resin layer is used in place of the thermoplastic polyimide resin, a double sided substrate or a multilayer substrate cannot be prepared without using an adhesive since the thermosetting polyimide resin does not have an adhesive function. Thus, the JP-A-61-22937 proposes a double sided substrate prepared by forming a thermoplastic polyimide resin layer on both surfaces of a thermosetting polyimide resin layer and press-bonding under heating a copper foil to the surface of each thermoplastic polyimide resin layer. However, since the double sided substrate has the problems as described above, such a substrate is not yet satisfactory.
Also, a method of adhering polyimide resin layers of two-layer substrates each comprising a copper foil and a thermosetting polyimide resin layer to each other through a thermoplastic polyimide resin layer may be considered but in such a case, a sufficient adhesion is not sometimes obtained at the interfaces of the both kinds of the resin layers as described above.
Furthermore, for electrically conducting metal layers or wiring circuits of the above-described double sided substrate or the multilayer substrate, through-holes are formed in the insulating resin layer (substrate) by a mechanical method using a drill, etc., a wet etching method using a chemical solution, or a chemical method using a photolithography by a photosensitive resin. Then, a complicated method of forming a thin metal layer (seed layer) on the inside wall of each through-hole thus formed by electroless plating or a vapor deposition, and further increasing the thickness of the thin metal layer by applying thereto elecrolytic plating to form a conducting path has been employed.
However, when the diameter of the through-hole is small, there is a possibility of causing an inferior formation of the thin metal layer on the inside wall of the through-hole and also the above-described production step becomes more complicated. In particular, it is not easy to visually confirm the thin film-form (layer-form) conducting path formed on the surface of the inside wall of the through-hole, and for confirming the formation of the conducting path, it is necessary to confirm it by contacting a probe for conducting path insepction with the land portions of the wiring circuits at both ends of the through-hole, and it is necessary to pay a close attention to the reliability of the electric conductivity.
Furthermore, in the formation of the through-hole, there is a limit on the fineness of the diameter of the through-hole in the case of using the drill or wet etching with a chemical solution, and thus the through-holes with a fine pitch are reluctant to form by the conventional techniques.
Also, in the case of using a photosensitive resin, a high aspect ratio is not obtained according to the kind of the resin composition employed and hence the material being used is limited. Also, in this case, the steps of a light exposure, a development, a pre-bake, an after-bake, etc., are required, which makes the formation step complicated.
In particular, if the diameter of the through-hole is too fine, a plating liquid being used for forming the conducting path does not sufficiently permeate into the through-hole, whereby not only a sure electrical conductivity is not obtained but also there are tendencies that the adhesion at the interface between the inside surface of the through-hole and the thin metal layer formed by plating is insufficient and also the electric resistance of the conducting path is increased.
Moreover, since the through-holes are formed after preparing the double sided substrate, the confirmation inspection of the preciseness of the formation of the through-holes is made in the latter half of the whole production steps, whereby the check of the formation of an inferior product is delayed, which is undesirable from the point of the production efficiency.
Also, in the case of using the above-described through-hole plating method, it is not so much expected to further improve the adhesive strength between the metal layers or wiring circuits formed on both surfaces of the substrate and the insulating resin layer, and according to the conditions of heating and press-adhesion at the preparation of the double sided substrate, it sometimes happens to cause peeling at the interfaces.